# Interpreting the Basic and Comprehensive Metabolic Panel

## Introduction

The basic metabolic panel (BMP) and comprehensive metabolic panel (CMP) are among the most frequently ordered laboratory tests. They provide essential information about electrolyte balance, renal function, glucose metabolism, and hepatic function. The pathologist must understand the analytic principles, reference ranges, clinical significance of abnormalities, and common sources of error.

## Components of the BMP

The BMP includes eight analytes: **Sodium (Na+)** with a reference range of 136-145 mEq/L, **Potassium (K+)** at 3.5-5.0 mEq/L, **Chloride (Cl-)** at 98-106 mEq/L, **Bicarbonate (CO2)** at 22-29 mEq/L, **Blood urea nitrogen (BUN)** at 7-20 mg/dL, **Creatinine** at 0.7-1.3 mg/dL for adult males and 0.6-1.1 mg/dL for adult females, **Glucose** at 70-100 mg/dL fasting, and **Calcium** at 8.5-10.5 mg/dL total.

| Analyte | Reference Range | Units |
|---|---|---|
| Sodium (Na⁺) | 136–145 | mEq/L |
| Potassium (K⁺) | 3.5–5.0 | mEq/L |
| Chloride (Cl⁻) | 98–106 | mEq/L |
| Bicarbonate (CO₂) | 22–29 | mEq/L |
| BUN | 7–20 | mg/dL |
| Creatinine | 0.7–1.3 (M) / 0.6–1.1 (F) | mg/dL |
| Glucose (fasting) | 70–100 | mg/dL |
| Calcium (total) | 8.5–10.5 | mg/dL |

## Additional CMP Components

The CMP adds hepatic and protein analytes to the BMP: **Albumin** (3.5-5.0 g/dL), **Total protein** (6.0-8.3 g/dL), **Total bilirubin** (0.1-1.2 mg/dL), **Alkaline phosphatase (ALP)** (44-147 IU/L), **AST (SGOT)** (10-40 IU/L), and **ALT (SGPT)** (7-56 IU/L).

![Basic and comprehensive metabolic panel analyte overview](images/bmp-cmp-overview.jpg)

## Electrolyte Interpretation

### Sodium Disorders

**Hyponatremia** (below 136 mEq/L) requires assessment of volume status, with common causes including SIADH, heart failure, and cirrhosis. Pseudohyponatremia from hyperlipidemia or hyperproteinemia should be excluded. **Hypernatremia** (above 145 mEq/L) reflects a water deficit and occurs in conditions such as diabetes insipidus or inadequate free water intake.

### Potassium Disorders

**Hypokalemia** (below 3.5 mEq/L) is commonly caused by diuretics, GI losses, or renal tubular acidosis. **Hyperkalemia** (above 5.0 mEq/L) is seen with renal failure, ACE inhibitors, and tissue destruction. Importantly, **pseudohyperkalemia** from hemolysis, prolonged tourniquet use, thrombocytosis, or leukocytosis must always be considered before acting on an elevated potassium result.

### Acid-Base Assessment

The **anion gap** is calculated as Na minus the sum of Cl and HCO3, with a normal range of 8-12 mEq/L. An elevated anion gap points to the causes remembered by the **MUDPILES** mnemonic: methanol, uremia, DKA, propylene glycol, isoniazid/iron, lactic acidosis, ethylene glycol, and salicylates. A non-anion gap metabolic acidosis suggests diarrhea, renal tubular acidosis, or normal saline infusion.

## Renal Function Markers

The **BUN/creatinine ratio** is normally about 10-20:1 and is elevated in prerenal azotemia and GI bleeding. The **eGFR** is calculated from creatinine using the **CKD-EPI equation**, which has been race-neutral since 2021. **Cystatin C** is an alternative filtration marker less affected by muscle mass and is recommended for confirmatory eGFR.

### Creatinine Pitfalls

The Jaffe method for creatinine measurement is subject to interference from ketones, cephalosporins, and bilirubin. **Enzymatic creatinine assays** are more specific and increasingly preferred. Low muscle mass can cause creatinine to underestimate true renal impairment.

## Calcium and Albumin

Total calcium must be corrected for albumin using the formula: **Corrected Ca = measured Ca + 0.8 x (4.0 - albumin)**. Ionized calcium is the physiologically active form and is preferred in critically ill patients. For hypercalcemia, primary hyperparathyroidism and malignancy together account for over 90% of cases.

![Algorithm for hyponatremia workup](images/hyponatremia-algorithm.jpg)

## Hepatic Panel Interpretation

A **hepatocellular pattern** features AST and ALT markedly elevated (more than 10 times the upper limit of normal) and suggests viral hepatitis, drug toxicity, or ischemia. A **cholestatic pattern** shows ALP elevated disproportionately to transaminases and points to biliary obstruction or drug reaction. An **AST/ALT ratio greater than 2** suggests **alcoholic liver disease**. An isolated elevated bilirubin with normal liver function tests should raise consideration of **Gilbert syndrome** (unconjugated) or hemolysis.

## Pre-Analytical Considerations

**Hemolysis** is the most common pre-analytical error and falsely elevates K+, LDH, and AST. **Fasting status** affects glucose and triglycerides. **Specimen handling** is important because delayed centrifugation causes K+ leakage from cells. **IV contamination** from drawing near an IV line can dilute or contaminate results.

## Critical Values

Critical values requiring immediate clinician notification include Na+ below 120 or above 160 mEq/L, K+ below 2.5 or above 6.5 mEq/L, glucose below 40 or above 500 mg/dL, calcium below 6.5 or above 13.0 mg/dL, and creatinine above 10.0 mg/dL (context-dependent).

![Critical value notification and communication workflow](images/critical-value-workflow.jpg)

## Clinical Pearls

The hemolysis index should always be checked before interpreting potassium results. Corrected calcium should be calculated whenever albumin is low, and ionized calcium is preferred in the ICU setting. The anion gap is a rapid bedside tool for classifying metabolic acidosis. eGFR equations have important limitations in extremes of body habitus, pregnancy, and acute kidney injury.

## References

1. Kraut JA, Madias NE. Metabolic acidosis: pathophysiology, diagnosis and management. *Nat Rev Nephrol*. 2010;6(5):274-285.
2. Inker LA, et al. New creatinine- and cystatin C-based equations to estimate GFR without race. *N Engl J Med*. 2021;385(19):1737-1749.
3. Lippi G, et al. Preanalytical variability: the dark side of the moon in laboratory testing. *Clin Chem Lab Med*. 2006;44(4):358-365.
4. Williamson MA, Snyder LM. Wallach's Interpretation of Diagnostic Tests. 11th ed. Wolters Kluwer; 2020.